Every primary aluminium producer, remelter or recycler of aluminium is faced with the fact that in all processes involving molten aluminium in contact with air dross is formed as the molten aluminium metal reacts with the furnace atmosphere [1]. By ‘aluminium’ in this context, we include aluminium alloyed with various alloying metals. ‘Dross’ as used herein refers to the mass of solid impurities floating on the surface of the molten aluminium metal in a furnace, comprising aluminium oxide and trapped recoverable aluminium metal. Dross generally represents from 1 to 10% by weight of the melt and depending on the process, may contain, on average, between 30 and 60% by weight free aluminium metal dispersed in an oxide layer, although higher and lower contents may occur [2][3]. Dross freshly removed from the furnace is of course hot, but most dross processing methods are practiced on dross that has been allowed to cool, often being transported long distances for processing.
Traditionally, the recovery of the aluminium metal has been performed using the rotary salt furnace (RSF) process. In the RSF process, an oil or gas fired furnace is charged with the dross, and a salt flux (up to 50% of the dross weight) is added. The salt protects the metal from the reactive atmosphere and facilitates agglomeration and separation of the metal, thereby increasing metal recovery. Using salt produces significant disadvantages such as increased costs, environmental hazards and safety hazards. The non-metallic by-product, referred to as the “salt cake”, is a mixture of aluminium oxides and aluminium nitrides, metal and salts. More than 1 ton of salt cake is produced for every ton of dross treated and is a growing environmental concern [4]. Stricter regulations, particularly in Europe, have resulted in the outlawing of landfill disposal of the salt cake. As such significant research and development activities have been conducted at different locations around the world with the objective of developing a salt-free process.
The Alcan Plasma Arc Dross Treatment Process
U.S. Pat. No. 4,960,460 discloses the Alcan plasma arc dross treatment process. Instead of using a gas or fuel burner, as is the case for the RSF, a plasma torch is used to provide the required heat for heating the charge in a rotary furnace. The torch is mounted on the charging door of the rotary furnace, which allows close control of the atmospheric composition. The plasma torch consists of two water-cooled internal electrodes separated by a small gap through which the process gas, such as air or nitrogen, is continuously injected. The cold dross is charged, the door is closed and the application of high voltage initiates an electric arc between the torch electrodes. The arc heats the gas to a very high temperature, and the charge is heated to 700-800° C. while the furnace is rotated. During heating of the dross by the plasma arc, which operates with air or nitrogen as the process gas, further oxides and nitrides are formed as the plasma gases react with some of the free metal contained in the dross. The rotation of the furnace provides mechanical stirring that ruptures the oxide film, freeing molten metal and improving metal recovery. The oxide portion of the dross, termed non-metallic product (NMP), is a greyish powder containing mostly alumina with variable quantities of aluminium nitride and magnesium oxide depending on the alloy composition [5]. This process requires high maintenance as the torch has to be removed periodically for electrode maintenance.
The Hydro-Quebec DROSCAR Graphite Arc Process
DROSCAR uses a DC electric arc, stretched and maintained between two graphite electrodes, to heat the charge above the aluminium melting point. The energy transfer mechanism is mainly radiation from the arc and conduction between the heated refractories and the charge. The furnace rotates during heating of the dross to provide mechanical stirring. The rotation also prevents formation of hot spots on the charge or refractories, and improves energy transfer. On completion of heating, the metal is tapped from the furnace through a side tap hole [6]. Since this process uses a graphite arc, there is no need for cooling water and maintenance is not as intensive as is the case for the plasma arc technology.
The ALUREC Process
The ALUREC process uses a rotary tiltable converter type furnace comprising an oxy-fuel burner and an exhaust gas port located on the same side of the furnace. This design produces high-energy efficiency and allows good control of the furnace atmosphere [4]. The oxy-fuel burner heats the converter refractory wall in a short time to about 1000° C. Through furnace rotation, the heat is transferred to the charge by conduction, and the heat is further distributed within the charge by mixing. Heat is also transferred through direct radiation from the flame to the charge. The metal is collected at the bottom of the converter, and a solid NMP floats on top. The metal is tapped separately from the NMP and can be returned directly into the melting or holding furnaces or cast into sows or T-ingots. The NMP is discharged through the converter mouth [6]. The exhaust gas from the oxygen-fuel burner does not contain nitrogen and the volume of exhaust gas is small. The reduced exhaust gas volume and the increased flame temperature results in a more energy efficient process.
The ECOCENT Process
In the ECOCENT process hot dross is fed without any fluxing salt additions into a converter where the relevant parameters for separation such as temperature and viscosity can be adjusted [7]. In addition, large lumps of dross are crushed into smaller pieces in order to improve the later separation of the metal. After homogenizing and adjusting the temperature the hot dross is poured as quickly as possible into a centrifuge. In the ladle or alternatively in the mould of the centrifuge the centrifugal forces are used for the separation of the metal from the aluminium oxide, the major constituents of the dross. As soon as the centrifuging is finished the liquid aluminium can be poured back into the furnace or can be used for ingot pouring. As this process utilises the inherent energy of the hot dross no additional energy input is required, resulting in less than 50% energy consumption as compared to more conventional dross processing techniques.
The DROSRITE Process
WO 97/39155 discloses the DROSRITE process for processing aluminium dross. Hot dross is charged to a pre-heated refractory-lined rotary furnace immediately after skimming from the aluminium holding furnace. The DROSRITE furnace is sealed and maintained under an argon atmosphere. The furnace is rotated, as it is necessary to gently tumble the charge. The tap hole is opened, and the metal is poured into the receiving vessel or ladle. A controlled amount of oxygen is then injected into the furnace cavity, burning some of the non-recoverable aluminium metal contained in the residue in order to increase the temperature to target value, typically in the range of 800-900° C., at which time oxygen injection is stopped. This process does not require any external energy input; process energy is extracted from the solid residue, stored in the furnace refractory wall, and released to the next batch of fresh dross.
The abovementioned dross processing processes either require the input of significant amounts of external energy or require complex and expensive apparatus. In the Ecocent and Drosrite processes energy requirements are greatly reduced, however the initial capital costs and running costs are high. Thus, there is clearly a requirement for an energy efficient dross processing process and apparatus which does not use fluxing salts or complex, expensive apparatus.